Luke Van Sickle, president of the prison guards' union at Lucasville, said the shadow of the riot is always present at the 1, 625-acre prison, where 1, 365 inmates are housed.
Shadow justice secretary Sadiq Khan told MPs that the current prison population - which has reached a new high - was about "3, 000" higher than when Mr Clarke took office.
Once a potent symbol of American frustration with the Vietnam War, the prison is now a tourist attraction, in the shadow of an upscale high-rise building owned by a Singapore businessman.